Respiratory systems provide breathable gas, such as oxygen, anesthetic gas and/or air directly to a patient's mouth, nose or airway to assist or facilitate breathing by the patient. A ventilator may be used as part of the respiratory system to drive the breathable gas to the patient through an inspiratory limb hose or conduit of a breathing circuit. The breathing circuit may include an expiratory limb hose or conduit to carry expelled air and other gas(es) from the patient back to the ventilator.
It is typically desired to warm and impart humidity to the breathable gas before it is provided to the patient. For that purpose, many respiratory systems include a humidification system having a chamber for holding water and a heater unit to which the chamber may be releasably mounted. The heater unit includes a heater, which may be comprised of one or more heating elements and a metal plate defining a hot plate. A wall of the chamber, such as the bottom surface of the chamber, is thermally conductive and in thermal contact with the hot plate of the heater, to thus heat the water in the chamber. The chamber may be manually refillable, or there may be a water source to selectively fill the chamber as it empties. The breathable gas is coupled to the chamber and is passed through the chamber to be heated and humidified. Examples of heater unit and chamber arrangements are shown in U.S. Pat. Nos. 6,988,497 and 5,943,473. The inspiratory limb carries the heated and humidified gas to the patient and the expiratory limb, if present, carries exhaled air and possibly other gases from the patient. Either or both of the inspiratory and expiratory limbs may be heated such as by heater circuits, which may be comprised of wires running through and along the hose or conduit interior. An example of a breathing circuit with heated limbs is shown in U.S. Pat. No. 6,078,730. In some settings, the limb(s) may not be heated.
In a typical heating arrangement, a temperature set point is established, such as with a thermostat or the like. As the temperature falls below the set point, power to the heater is turned on or increased to bring the temperature back up toward the set point. If the temperature moves above the set point, power to the heater element is reduced or turned off to allow the temperature to drop down toward the set point. In many applications, it is possible to provide more accurate control of the temperature by use of a more sophisticated control based on PID or proportional-integral-derivative control. As is well known, feedback controls operate by obtaining an input signal indicative of the current state of a variable of the system, obtaining a difference or error signal between the input signal and a set point representing the desired value, and outputting a correction signal to be used by the system to drive the system toward the set point. With PID feedback control, the feedback loop involves computations involving empirically or otherwise determined coefficients for each of the proportional, integral and derivative aspects of the error signal.
The coefficients of PID feedback control are typically determined in relation to the nature of the variables which can affect behavior of the system. In the context of a respiratory system, the input value is taken from measured temperatures, and the output is used to regulate power to the heater in an effort to drive the temperature toward the set point. Variables such as flow rate and/or humidity level are generally not monitored or known for purposes of the control, yet they can have an impact on reliable control of the heating element(s) and can affect the temperature and humidity level of the gas reaching the patient. By way of example, the coefficients may be predetermined on the assumption that the flow rate will always be within a certain range. Should the flow rate actually be lower, less heat will be removed from the chamber as the gas flows therethrough than was anticipated and may result in large swings in the resulting temperature, including temperature excursions undesirably above or below the set point. Similarly, if the flow rate is actually higher than the assumed flow rate, more heat will be removed than anticipated, and the control may not be able to get the temperature to the desired set point. The challenge is thus to provide reliable PID feedback control in reliance on temperature measurements in an environment where variations in flow rate and/or humidity level are not known or cannot be directly determined, yet can impact performance of the system.